1. Field of the Invention
This application relates to apparatus and methods for recovering plastic from commingled plastic and fiber waste, and more particularly, to a dry process for removing paper and other contaminants from precleaned hydropulp polywaste comprising polyethylene. A preferred use for the recovered plastic is in the production of extruded composite building materials comprising plastic and wood fiber.
2. Description of Related Art
In the reclamation of plastic from various sources, the use of magnets, screens, shaker tables, and the like, is well known for removing tramp materials such as ferrous and non-ferrous metals, rocks, wood chunks, etc. The separation of lighter contaminants such as paper, undesirable comingled plastics, nylon tape strips, and the like, from reclaimed plastic has been more difficult. While cyclones are sometimes useful for separating lighter materials, they too are often inadequate when the contaminants are similar in size and weight to the plastic sought to be reclaimed.
One major source of reclaimable polyethylene suitable for use as a feedstock in manufacturing extruded composite building materials made of plastic and wood fiber is baled polywaste from hydropulping mills. Hydropulping mills recover paper fiber, often bleached paper fiber having a comparatively high value, from plastic-coated paper scrap such as that used to make milk cartons, plastic-coated paper plates, and other products. However, because reclaimed polyethylene contaminated with a substantial amount of paper fiber is generally unsatisfactory for use by manufacturers of plastic-coated paper products, the plastic-containing waste material from the hydropulping mills is dewatered, baled and sold as scrap. In the past, the amount of paper remaining in the precleaned polywaste from hydropulping mills often exceeded 20% by weight. As paper recycling technology has improved, the paper content of the hydropulping polywaste has fallen, frequently to levels of 15% or below.
The presence of minor amounts of paper fiber, such as about 5% or less, in reclaimed plastic used for manufacturing composite wood and plastic building materials is not detrimental to the resultant product, provided that the moisture content is maintained at a satisfactorily low level. In the past, particularly when the paper content of plastic waste obtained from hydropulpers, from post-consumer plastic bags and from other sources was higher, one method use for separating the plastic and paper was to place the combined waste in a water tank, sometimes in the presence of flocculants. Because paper fibers readily absorb water, they tend to sink in water separation tanks, whereas polyethylene plastic scrap will float and can be skimmed from the top. Nevertheless, the use of such "wet" processes has several disadvantages including, for example, the capital and operating costs associated with installing and operating water circulation pumps and filtration systems, pH and flocculant control, and compliance with wastewater treatment regulations. Moreover, the higher water content of any paper recovered from the water separation tank together with the plastic can cause problems during extrusion unless the water is first removed by thorough drying.
Prior art wet systems are used to prewash and clean highly contaminated plastic and paper mixtures containing up to 50% paper or fibrous waste. Assignee of the present invention also owns U.S. Pat. Nos. 5,084,184, 5,100,545 and 5,351,895, which disclose a method and related apparatus for use in recovering plastic from contaminated recycle scrap containing plastic, paper and other tramp materials through use of sink/float water separation techniques.
An efficient and economical process is therefore needed that can be used to separate paper and tramp contaminants from reclaimed plastic waste, particularly hydropulp polyethylene waste, without the need for water separation tanks or related equipment. An effective system and method for recovering plastic from baled hydropulp by a dry process are needed to avoid the drying time and expense that are otherwise required to remove moisture from the plastic and paper recovered through use of the wet process. One such particularly effective system and method are disclosed herein.